Organic solar cell research has developed during the past 30 years, but especially in the last decade it has attracted scientific and economic interest triggered by a rapid increase in power conversion efficiencies. This was achieved by the introduction of new materials, improved materials engineering, and more sophisticated device structures. Today, solar power conversion efficiencies in excess of 3% have been accomplished with several device concepts. Though efficiencies of these thin-film organicdevices have not yet reached those of their inorganic counterparts (η ≈ 10–20%); the perspective of cheap production (employing, e.g., roll-to-roll processes) drives the development of organic photovoltaic devices further in a dynamic way. The two competitive production techniques used today are either wet solution processing or dry thermal evaporation of the organic constituents. The field of organic solar cells profited well from the development of light-emitting diodes based on similar technologies, which have entered the market recently. We review here the current status of the field of organic solar cells and discuss different production technologies as well as study the important parameters to improve their performance.

https://www.cambridge.org/core/services/aop-cambridge-core/content/view/S0884291400094498

Organic solar cells: An overview

The emergence of semiconductor nanocrystals as the building blocks of nanotechnology has opened up new ways to utilize them in next generation solar cells. This paper focuses on the recent developments in the utilization of semiconductor quantum dots for light energy conversion. Three major ways to utilize semiconductor dots in solar cell include (i) metal−semiconductor or Schottky junction photovoltaic cell (ii) polymer−semiconductor hybrid solar cell, and (iii) quantum dot sensitized solar cell. Modulation of band energies through size control offers new ways to control photoresponse and photoconversion efficiency of the solar cell. Various strategies to maximize photoinduced charge separation and electron transfer processes for improving the overall efficiency of light energy conversion are discussed. Capture and transport of charge carriers within the semiconductor nanocrystal network to achieve efficient charge separation at the electrode surface remains a major challenge. Directing the future resear...

http://wiki.phy.queensu.ca/shughes/images/d/d1/Quantum_Dot_Solar_Cells._Semiconductor_Nanocrystals_as_Light_Harvesters.pdf

Quantum Dot Solar Cells. Semiconductor Nanocrystals as Light Harvesters

The increasing energy demand in the near future will force us to seek environmentally clean alternative energy resources. The emergence of nanomaterials as the new building blocks to construct light energy harvesting assemblies has opened up new ways to utilize renewable energy sources. This article discusses three major ways to utilize nanostructures for the design of solar energy conversion devices:  (i) Mimicking photosynthesis with donor−acceptor molecular assemblies or clusters, (ii) semiconductor assisted photocatalysis to produce fuels such as hydrogen, and (iii) nanostructure semiconductor based solar cells. This account further highlights some of the recent developments in these areas and points out the factors that limit the efficiency optimization. Strategies to employ ordered assemblies of semiconductor and metal nanoparticles, inorganic-organic hybrid assemblies, and carbon nanostructures in the energy conversion schemes are also discussed. Directing the future research efforts toward utiliza...

Meeting the Clean Energy Demand: Nanostructure Architectures for Solar Energy Conversion

A surface science perspective on TiO2 photocatalysis

Organic electronic materials are of interest for future applications in solar cells. Although results for single layer organic materials have been disappointing, high photocurrent quantum efficiencies can be achieved in composite systems including both electron donating and electron accepting components. Efficiencies of over 2% have now been reported in four different types of organic solar cell. Performance is limited by the low red absorption of organic materials, poor charge transport, and low stability. These problems are being tackled by the synthesis of new materials, the use of new material combinations, and optimisation of molecular design, self assembly and processing conditions to control morphology. Power conversion efficiencies of over 5% are within reach, but the fundamental physics of organic donor–acceptor solar cells remains poorly understood. Within the last 18 months, power conversion efficiencies of over 2% have been achieved in four different types of organic solar cells. All are composite systems including electron donating and electron accepting components. Performance is limited by weak absorption in the red, poor charge transport, and low stability, but improvements are available through optimisation of materials and device structures.

Organic photovoltaic films

A detailed study of the optoelectronic processes occurring in ultrathin ${\mathrm{TiO}}_{2}$ nanocrystal-conjugated polymer, poly(p-phenylene vinylene) (PPV) composites is presented Composites of ultrathin films (about 100 nm) are studied spectroscopically and as the active medium in photovoltaic devices of the structure (Al/composite/indium tin oxide) By varying the weight ratio of the nanocrystals and using results of photoluminescence efficiency, photocurrent, and photovoltaic measurements and time-resolved microwave conductivity, we are able to construct a well-defined picture of the relevant processes in the composite: including exciton dissociation, charge transport, and recombination We combine the experimental results with a hopping model for charge transport in a nanocrystal lattice (random walk or biased random walk) to determine the probability of electron collection as a function of distance from a collecting contact in a nanocrystal lattice The combined results indicate that most photogenerated excitons lead to charge separation at the interface between the polymer and the nanocrystals above 20-wt % ${\mathrm{TiO}}_{2}$ nanocrystals, but the electron collection efficiency in photovoltaic devices is limited by fast recombination The transport model indicates that even for a relatively long recombination time and a well-ordered nanocrystal lattice, most of the collected charge will originate from the first several nanocrystal layers at the electrode rather than from sites throughout the film, due to the recombination process We also argue that the existence of a photocurrent in these and related devices is not necessarily evidence of charge transport through a network of the nanocrystals (or other component), as the quantum yields can be accounted for by interfacial charge transfer at the contact alone Quantum yields for collecting charge following direct band-gap excitation of the ${\mathrm{TiO}}_{2}$ are more than a factor of 10 larger than for excitation into the polymer, suggesting that either hole transfer to the polymer, or some preceding process, is rate limiting and much slower than the corresponding process following polymer excitation We also examine the key differences between the mechanisms underlying conjugated polymer:nanocrystal devices and conventional, silicon pn devices

/pdf/exciton-dissociation-charge-transport-and-recombination-in-3vq0bw4o7p.pdf

J. S. Salafsky

Papers

Exciton dissociation charge transport, and recombination in ultra thin, conjugated polymer-TiO2 nanocrystal intermixed composites